Added sys_physzero library call for corresponding system call; modified

system-printf() so it returns number of characters printed (for use in
smart formatting)

lib/syslib/sys_physzero.c

@@ -0,0 +1,16 @@
+#include "syslib.h"
+
+PUBLIC int sys_physzero(phys_bytes base, phys_bytes bytes)
+{
+/* Zero a block of data.  */
+
+  message mess;
+
+  if (bytes == 0L) return(OK);
+
+  mess.PZ_MEM_PTR = (char *) base;
+  mess.PZ_COUNT   = bytes;
+
+  return(_taskcall(SYSTASK, SYS_PHYSZERO, &mess));
+}
+